problems designing filter

Discussion in 'The Projects Forum' started by mantaka, Mar 10, 2015.

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  1. mantaka

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    Nov 27, 2014
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    i am designing a narrow band pass filter using ua741. when i simulate the circuit on pspice i get exactly what i want.but after implementation i cant vary frequency of the signal by varying resistance.it only varies amplitude of the signal.now what is the problem here i can't figure it out.my circuiis this:
    seh_2polebandpass.png
     
  2. MikeML

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    To keep the filter cutoff frequency, you cannot vary just one component.

    Put a pot between Vout and gnd as a voltage divider. That way the filter cutoff is not effected by adjusting gain...
     
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  3. mantaka

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    Nov 27, 2014
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    thank you.what range of pot should i use?amd if i use pot then can i vary frequency of signal?please let me know
     
  4. MikeML

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    Are you trying to make the filter tune-able by varying either the resistances or the capacitors?

    Selecting a pot to adjust the amplitude of a signal requires knowing the input impedance of the following stage, and to a lesser extent, the output impedance of the upstream stage...
     
  5. crutschow

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    You cannot vary the bandpass frequency of such a filter with a single pot. All three resistors need to vary.
    Your simulation will show you that.
     
  6. mantaka

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    i want 2 vary the frequency of the signal but by varying pot it only varies amplitude.i did simulation and i got exact result i wanted but in practical i can't
     
  7. mantaka

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    Nov 27, 2014
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    i did simulation and by varying R2 i can vary the cutoff frequency.i did ac analysis.but after implementation it doesn't work.now what is the problem i can't figure it out.please hep me
     
  8. crutschow

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    Post the circuit you simulated with values and power supplies.
     
  9. mantaka

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    Nov 27, 2014
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    upload_2015-3-12_19-47-21.png upload_2015-3-12_19-48-25.png upload_2015-3-12_19-48-36.png
    here i applied 4k sinewave signal at input and designed to get this value.during simulation i got the exact value but in lab it doesn't vary frequency.plz help me.now what to do.or wht this doesn't work in lab?
     
  10. MikeML

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    We cannot read the values of the R and Cs.

    You seem not to understand this very important point:
    The MFB filter you show is not tuneable by changing just one resistor, or just one capacitor. Changing just one resistor, or one capacitor in isolation changes the center frequency, and the shape of the filter. This likely is not useful or desirable...

    In order to shift the center frequency of the filter, keeping the bandwidth and other attributes, you must change all three resistors at the same time. (or change the two capacitors at the same time).
     
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  11. mantaka

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    Nov 27, 2014
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    thank you very much.i did this cause i got this circuit from a link and there was instruction about this circuit to change only R2. that's why i did this.will u can plz help me by showing me a good circuit diagram of narrow band pass filter range of 1kHz.plzzzzz
     
  12. MikeML

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  13. mantaka

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  14. mantaka

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    can anyone plz help me by informing me why ua741 doesn't give exact results.i have to give answer to my teacher tomorrow but i don't know the answer.plz plz plz inform me
     
  15. MikeML

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    You will have to explain what the shortcoming is...
     
  16. mantaka

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    Nov 27, 2014
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    sorry i didn't understand what u said.i can't find the problems with my circuits.but i have to explain it to my teacher.this is a portion of my thesis.so plz plz plz help me.what should i say about the problems of my circuit.plz .
     
  17. crutschow

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    But what, exactly, is the problem?
     
  18. mantaka

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    Nov 27, 2014
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    problem is i can't vary cut off frequency
     
  19. MikeML

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    I give up!
     
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